Computer Systems 1 Fundamentals of Computing Von Neumann & Fetch Execute Cycle

Computer Systems 1 ( ) Von-Neumann & Fetch Execute Von-Neumann Model Fetch Execute Cycle  Get It  Do It Follow these instructions…. Von-Neumann Bottleneck  Problems  RAM attempts to save the world! VRAM  Clean the Windows

Computer Systems 1 ( ) Von Neumann Model Logical Structure of the computer system  Routes of data transfer during processing

Computer Systems 1 ( ) Fetch Execute Cycle Defines how instructions are retrieved and carried out inside the processor (CPU) Sometimes called the Instruction Cycle or Automatic Sequence Control START Fetch next instruction from memory to CIR Increment PC Execute instruction in CIR STOP? END no yes

Computer Systems 1 ( ) Fetch Execute Cycle Fetch Stage  Copy contents of PC into MAR Value of PC presented via the address bus  Increment PC (point to next instruction)  Copy instruction from MBR into CIR via data bus Instruction retrieved from memory Placed in CIR START Fetch next instruction from memory to CIR Increment PC Execute instruction in CIR STOP? END no yes

Computer Systems 1 ( ) Fetch!

Computer Systems 1 ( ) Fetch Execute Cycle Execute Stage  Decode instruction from CIR  Run instruction in CIR May require getting data from memory  Unless current instruction is STOP, repeat cycle START Fetch next instruction from memory to CIR Increment PC Execute instruction in CIR STOP? END no yes

Computer Systems 1 ( ) Execute...

Computer Systems 1 ( ) MIPS & Hertz MIPS = Millions of Instructions per Second  How many instructions a processor can carry out each second  Old form of measurement Inaccurate Some instructions take longer than others Hz = number of complete cycles per second MHz = Millions of cycles per second  In a processor a cycle is when the state of the control lines are changed

Computer Systems 1 ( ) CPU Instructions Instruction Set  The types of instruction that a particular machine can execute  The instructions that are carried out during the Fetch Execute Cycle  Types of instruction: Arithmetic and logical calculations on data Input and output of data Changing the sequence of program execution Transferring data between memory and CPU registers Transferring data between CPU registers

Computer Systems 1 ( ) CPU Instructions Instructions are split into two parts:  Opcode (Operation Code)- the operation to be carried out  Operand- The data upon which the operation should be performed Different manufacturers - different instruction sets  Can vary in architecture  Functions will often be the same or similar but may vary in name  More advanced functions may be present Likely to vary between manufacturers  e.g.- Intel instruction set Vs. AMD instruction set

Computer Systems 1 ( ) Assembly Language Is at a level below programming languages  Eg.- C++, Java, Pascal Assembly language is converted into machine code  Machine code is raw data that would take ages for a human to decipher  This is the data and instructions which is used by the Fetch Execute Cycle

Computer Systems 1 ( ) Assembly Language Programs or sequences can be written in assembly language  Which is what is effectively done when we compile a C++ program Why write in assembly language?  Faster (direct) access to CPU  Some programs need to be written to operate at a lower level E.g.- Device Drivers

Computer Systems 1 ( ) A simple Assembly program (Honest!) org 100h mov dx,msg mov ah,9 int 21h mov ah,4Ch int 21h msg db 'Hello, World!',0Dh,0Ah,'$'

Computer Systems 1 ( ) A simple Assembly program (Honest!) org 100h mov dx,msg mov ah,9 int 21h Tells the compiler (NASM) the program will be loaded at memory address 100h Moves the address of our message (msg) into a register which is known as the DX Register (Data Register) Moves the value 9 into a register called the AH Register ‘int’ calls an ISR (interrupt service routine) “DOS Services” this is combined with contents of AH (9) to determine that we want to output a message– contents of DX (msg)

Computer Systems 1 ( ) A simple Assembly program (Honest!) mov ah,4Ch int 21h msg db 'Hello, World!',0Dh,0Ah,'$' Effectively tells the processor to stop (combines int 21h with contents of AH ). Otherwise it will try to fetch and execute the next instructions it comes to msg is a variable name (the name of out message string) db is an instruction to the compiler to use the information the follows as data Then out message ‘Hello, World!’ (note: ‘ ‘ marks) 0Dh, 0Ah – performs carriage return and line feed $ terminate string output – (int 21h & 9 in ah requirement)

Computer Systems 1 ( ) A simple Assembly program (Honest!) OK, so what does it actually do?  Output “Hello, World!” to the screen How?  Type the program into a text document and call it ‘hello.asm’  Use the NASM program This is used to compile assembly language programs  Rename the produced file as type COM ren hello hello.com  Run the program hello

Computer Systems 1 ( ) A simple Assembly program (Honest!)

Computer Systems 1 ( ) Von-Neumann Bottleneck Originally CPU & RAM ran at similar speeds  CPU development began to increase faster than RAM Fundamental problem:  CPU is faster than RAM Attempt to solve the problem:  Sophisticated RAM technologies

Computer Systems 1 ( ) Memory (RAM) Many different types of RAM  SDRAM Synchronous Dynamic RAM Synchronises with processor buses Max. approx 133 MHz Contemporary  DDR SDRAM Double Data Rate SDRAM Transfers data on both sides of the clock cycle Effectively doubles transmission rate

Computer Systems 1 ( ) Memory (RAM) Older methods:  EDO DRAM Extended Data Out DRAM  Gets the next block of memory while current block is being sent to the processor  BEDO DRAM Burst EDO RAM  Only synchronise with CPU for short time (‘bursts’) Processes four memory addresses in one go  Only supports max. 66MHz processor buses

Computer Systems 1 ( ) Memory (RAM) FPRAM  Fast Page RAM  “Page Mode Memory”  Dynamic RAM  Allows faster access to adjacent memory locations Does not always store complete addresses NVRAM  Non Volatile RAM  Retains it’s contents when power is switched off  Powered by a battery  Or uses an EEPROM chip Electrically Erasable Programmable ROM Combination of SRAM and EEPROM chips  SRAM is an NVRAM derivative

Computer Systems 1 ( ) Video Memory (VRAM) WRAM  Windows RAM Windows are large blocks of memory Supports two paths to transport data  Sends data for display as new information is being sent to the graphic adapter’s memory  Same principle as standard VRAM  Faster than ordinary VRAM because of Windowing SGRAM  Synchronous Graphic RAM Dynamic RAM Synchronise with processor buses up to 100 MHz Capable of opening two memory pages at once  Simulates dual data transmission of VRAM and WRAM Better than standard VRAM

Computer Systems 1 ( ) CS1: Week 8 What you know now:  Von-Neumann & Fetch Execute Cylcle  Stages of the F/E Cycle Retrieval Execution  Performance measurement  Instruction sets  Bottleneck remedies Types of RAM  Types of VRAM